1. Field of the Invention
The present invention relates to a crucible for the treatment of molten silicon, to the manufacture of such crucible and to the use of such crucible for treating molten silicon.
2. Description of the Related Art
Nowadays, the demand for high purity silicon has significantly increased. The applications of high purity silicon in the generation of photovoltaic energy are widely spread. Successive energy crises have strengthened that need.
The object of the present application is a vessel used for the treatment of molten silicon. Such treatment can consist in the silicon crystallization, either by directional solidification or by drawing of a crystal from a molten bath. The treatment can also consist in a metallurgical treatment intended for the production of very highly pure silicon or one of its alloys. It can thus consist in a metallurgical treatment of alloys or ores aiming at eliminating certain impurities.
For such kinds of applications, it is well known to use crucibles from quartz or based on other materials essentially constituted of silicon dioxide (see for example the document DE-C-962868). Indeed, since the main constituent of the crucible is silicon under the form of one of its oxides, the contamination risk by other chemical compounds is strongly reduced. The quartz crucibles present however the major drawback of being attacked by molten silicon, with the consequence that the solidifying silicon ingot tends to adhere to the walls of the quartz crucible. Since the quartz and silicon have different coefficients of thermal expansion, very significant mechanical stresses can be generated either inside the ingot resulting in crystallization defects, or in the crucible walls resulting into the crucible cracking. Further, the silicon ingot once solidified strongly adheres to the crucible walls and it is practically impossible to extract the ingot without destroying the crucible or at least seriously damaging it.
The quartz and certain silica derivatives are also known to be subject to crystallographic phase changes during their thermal cycling. These crystallographic phase changes induce very high mechanical stresses within the crucible walls. Further, they can also induce changes of density and, therewith, of thermal conductivity within the wall, leading to problems of losses of homogeneity of the transmission or withdrawal of energy towards or from silicon. So far, this crucial problem has not found a solution likely to be industrially carried out.
Moreover, at the temperatures of use, quartz is subject to geometrical changes. These geometrical changes are relatively difficult to manage since the treatment furnace wherein the vessel containing molten silicon stands must heat in a fully controlled way the quantity of silicon being treated. Any deformation of the vessel wall induces a loss of homogeneity of the transmission of withdrawal of energy towards or from silicon which adds to the loss of geometry of the silicon ingot during the crystallization.
This problem has been partly remedied by reinforcing the external walls of the quartz crucible by carbon plates, more particularly by graphite plates.
Such carbon plates, and more particularly graphite plates are widely used in all kinds of processes carried out at high temperature due to their excellent resistance to thermal stresses for quite long periods. For example, graphite crucibles have been used to receive a germanium bath during the carrying over of a process of crystal drawing according to the Czochralski method. So far, it has however not been possible to use such graphite crucibles for the treatment of silicon since, at high temperature, the molten silicon bath attacks the graphite walls and forms silicon carbide, the presence of which being incompatible with the required purity. According to the currently used technique, as indicated hereabove, the different processes of treating silicon at high temperature take place in quartz or other silica based materials crucibles of which the walls have been reinforced with carbon, more particularly graphite, plates.
This technique either is not problem-free. It is indeed well known that the gaseous phases, in the neighboring of the molten silicon bath, influence the formation of an equilibrium between the silicon vapor escaping from the molten silicon bath and the carbon monoxide atmosphere prevailing in the furnace. Reactions are observed as well on carbon or graphite as in the silicon bath, leading to a change of physical and mechanical properties.
Starting from the same concept consisting of avoiding introducing other constituents than silicon, it has also been proposed in the state of the art to use silicon nitride crucibles. Therefore, document WO-A1-2004/016835 discloses a crucible essentially constituted of silicon nitride. Even though some of the properties of this crucible are satisfactory, its price makes its use currently unrealistic. Further, it has been reported that these crucibles are also sensitive to deformation at high temperature.
The applicant has thus set as an objective the provision of a vessel for the treatment of molten silicon which would not present the drawbacks observed in the prior art. In particular, it would be desirable that the crucible could be used a certain number of times without any significant degradation of its physical integrity. Moreover, the thermal conductivity properties of the crucible in question should not change in the course of its use; in other words, that the material be not sensitive either to deformation or to crystallographic phase changes. Eventually, it is necessary that the crucible be not a source of silicon pollution.